1. Field of the Invention
The present invention relates to electroconductive materials and to positive electrode materials for lithium ion secondary batteries using the same.
2. Description of Related Art
A glass based on vanadium containing vanadium pentoxide as a main component is known to be a glass semiconductor that conducts electricity and has been studied to be adopted typically to thermistors.
Japanese Examined Patent Application Publication (JP-B) No. Sho 42(1967)-24785 (Document 1) discloses a vitrified resistive material composed of a glass composition containing 50 percent by mole or more of vanadium pentoxide, containing phosphorus pentoxide and barium oxide, and further containing cerium oxide, tin oxide and lead oxide as added.
Japanese Examined Patent Application Publication (JP-B) No. Sho 39(1964)-9140 (Document 2) discloses a thermistor made of a glass containing 70 percent by mole or more of vanadium pentoxide and 5 to 15 percent by mole of phosphorus pentoxide and further containing 13 percent by mole or less of copper oxide.
Japanese Patent No. 3854985 (Document 3) discloses a glass semiconductor having a specific resistance of 101 to 104 Ω·cm at room temperature. This glass semiconductor is prepared by heating a glass based on an oxide containing vanadium, barium and iron at a temperature equal to or higher than its glass transition temperature and equal to or lower than its crystallization temperature to reduce the strain of a glass skeleton.
FIG. 1 shows a model for a crystal structure of vanadium pentoxide (V2O5). V2O5 is an orthorhombic crystal, has a layered structure composed of VO5 pyramids as illustrated in FIG. 1. And V2O5 has been investigated to be adopted to an active material for nonaqueous electrolyte secondary batteries.
Japanese Patent No. 2973830 (Document 4) discloses a secondary battery that shows satisfactory cycling performance at a high energy density, because this secondary battery includes a positive electrode active material having coexisting two phases of an amorphous phase and a crystalline phase composed of a multiple oxide of vanadium and lithium or a first-row transition metal.
Japanese Patent Application Laid-Open (JP-A) No. 2007-42618 (Document 5) discloses a battery using a glass ceramic including a glass containing a highly electroconductive lithium-containing iron vanadium phosphate. This battery has improved rate properties and improved low-temperature properties.
Japanese Patent Application Laid-Open (JP-A) No. 2009-16277 (Document 6) discloses an electrode active material mainly containing at least one of a glass and a glass ceramic each composed of a lithium-containing iron vanadium phosphate containing at least one of Co, Mn and Ni; and a secondary battery using the electrode active material, which shows high discharge rate properties and a high energy density.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) (JP-A) No. 2005-506272 (Document 7) discloses an active material which is a lithium vanadium oxide corresponding to Li1+xV3O8 in which 0.1≦x≦0.25, and which has a monoclinic crystal structure; and also discloses a battery using the active material, which shows significantly improved capacity and cycling performance.
However, the glass semiconductor material disclosed in Document 3 shows a specific resistance of 200 Ω·cm which is excessively high as an electrode material or solid-state electrolyte, and there is a need of providing a material having a further lower specific resistance.
When the electrode active material disclosed in Document 5 is used as an active material of a nonaqueous secondary battery and is annealed to have a specific resistance minimized to 290 Ω·cm or less, the resulting secondary battery shows a decreasing battery capacity with a decreasing specific resistance. Independently, there has been provided no electrode active material that satisfies requirements to have higher capacity as battery typically for battery cars. Accordingly, investigations on active materials composed of oxides are actively made even now.
The present invention has been made to solve such problems in known techniques, and an object of the present invention is to reduce the specific resistance of an electroconductive material and to improve the function typically of an electrode material, a solid-state electrolyte, or a sensor such as a thermistor using the electroconductive material.
Another object of the present invention is to provide an electroconductive material using a glass based on vanadium which can give a battery that maintains a satisfactory battery capacity even when it has a reduced specific resistance.